Phase transformation of hydrogenated nanocrystalline silicon (nc-Si:H) films was essential in the solar cells industry. Therefore, the plasma chemistry monitoring of in-situ diagnostics optical emission spectroscopy (OES) of plasma-enhanced chemical vapour deposition (PECVD) is deemed crucial for investigating the phase transformation. The proposed PC2-OES algorithm can be used to monitor the PECVD process health condition of the crystallisation rate at different hydrogen dilution ratios. Principal component analysis (PCA) was performed to distinguish the crucial role of nc-Si:H emission characteristics of plasma chemistry and the resultant crystallisation rate. Measurement results revealed that OES spectra characterisation confirmed that the crystallisation rate index (Hα*/SiH*) was highly correlated to hydrogen dilution ratio (R) and phase transformation of nc-Si:H film. The proposed PCA-based evaluation method will provide valuable information to reflect consistently the crystallisation rate of deposited films with different hydrogen dilution ratio and possibly other processing parameters of mass flow rate and applied power density.